Back in the day, your ancestors used to enjoy fresh, creamy, delicious milk straight from the udder. They also kept coming up with great ideas for other foods to make out of milk, like butter, cheese, and ice cream.
More recently, dairy scientists had another spectacular idea: low-fat milk! Fat's bad, right? By extension, low-fat milk -- 2%, 1%, and skim -- was supposed to be better for us. But that turned out to be just a bunch of bullshit... or cowshit, as the case may be.
Where does skim milk come from, anyway?
Still coming from cows, don't worry. But there are a few steps with long names between the cow and the consumer: pasteurization, homogenization, and separation. Some people argue that we should skip all of these and campaign for hippie-friendly "raw milk." You can think of it as trendy, or you can acknowledge that when you skip the pasteurization and homogenization, you're really getting something that's potentially disease-causing and lumpy -- whatever floats your boat.